Haskell: Type Category: Multi-inheritance example

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE StandaloneDeriving #-}

class (Eq a, Show a) => C a where
 getHashCode :: a -> Integer
 getHashCode obj = 123


type Id = Int
type Name = String 

data Employee = Employee Id Name deriving C

No instance for (Eq Employee)
 arising from the'deriving' clause of a data type declaration
 Possible fix:
 use a standalone'deriving instance' declaration ,
 so you can specify the instance context yourself
 When deriving the instance for (C Employee)
Failed, modules loaded: none.

class (Eq a, Show a) => C a where

I started to know that we can use type classes Implement multiple inheritance. I have written small Haskell code, but I can’t figure out the problem.

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE StandaloneDeriving #-}

class (Eq a, Show a) => C a where
 getHashCode :: a -> Integer< br /> getHashCode obj = 123


type Id = Int
type Name = String

data Employee = Employee Id Name deriving C

When I tried to load the above code, I received the following error. Any help with this.

No instance for (Eq Employee)
 arising from the'deriving' clause of a data type declaration
 Possible fix:
 use a standalone'deriving instance' declaration,
 so y ou can specify the instance context yourself
 When deriving the instance for (C Employee)
Failed, modules loaded: none.

I searched Google for a while, but I couldn’t find multiple inheritance Good example. If you provide some information, such as multiple inheritance in Haskell, it will be helpful.

Reference: https://www.haskell.org/tutorial/classes.html

Talk

class (Eq a, Show a) => C a where

< p>It does not mean that the types that implement C automatically implement Eq and Show, which means that they must first implement Eq and Show before they can implement C.

A class in Haskell is different from a class in Java. It is closer to the interface, but it cannot be used in the same way (and should not be used). Haskell does not actually have the concept of inheritance or classes in the OOP sense, because it is not an OOP language.

However, If you want to automatically have Eq and Show instances for the type, just add them to the deriving clause of the data type.